For the last 10 days i walked 3.2 miles in average, some days 4+ miles, some days 5+. before walk i was 144 lbs, now 147 lbs. no diet increase. why?

Weight gain : Gaining and losing weight is generally a factor of what goes in(food)and what goes out (exercise). If diet is exactly the same, honestly nothing has changed, then causes could relate to water retention.However, i encourage you to get a free app like lose it or myfitnesspal and record everything that crosses your lips.This will help you assess your overall caloric intake and expenditure. Good luck!